Partitions which part liquid phases or gas phases from one another and thus allow exchange processes are described as membranes, which themselves are distinct phases built up from substances other than those of the phases which are to be separated. Membranes can be liquid or solid. Within the scope of the present invention, solid membranes of synthetic polymers are considered, and it should generally be assumed that those polymers are suitable for preparing them which can also be used for the production of films.
Depending on the intended uses, films should be impermeable to gases or liquids, whereas membranes allow a mass transfer between the phases separated by the membrane. The requirements to be met by the permeability differ depending on the type of mass transfer, such as osmosis, dialysis, ultrafiltration, electrodialysis and the like.
Membrane formation can be effected in different ways. More frequently, this is done via a polymer solution in a suitable solvent.
This membrane formation involves either evaporation or withdrawal from immersion in a precipitant (phase separation). In a suitable system, a phase separation can also take place as a result of cooling, a solvent-rich phase and a solvent-lean phase forming first, of which the solvent-lean phase solidifies on further cooling (German Offenlegungsschrift No. 2,737,745, German Offenlegungsschrift No. 2,833,493).
In the process for producing shaped articles having pores, according to German Patent No. 3,327,638, a porous polyamide-6 hollow fiber has already been produced from a mixture of polyamide-6, .epsilon.-caprolactam and polyethylene glycol 300. Forming took place at a nozzle temperature of 210.degree. C. The spinning solution was homogeneous and of low viscosity and was therefore extruded into a U-shaped cooling tube, in which the mechanical loading, to which the polymer mixture is exposed up to the time of solidification starting, that is to say the start of dimensional stability, is kept small.
In the Loeb-Souriragan process, a portion of the solvent is, after casting of the polymer solution, first evaporated and the polymer is then solidified by immersing the solution into a non-solvent. Low-boiling solvents are therefore necessary for this purpose. Aprotic solvents such as dimethylformamide, dimethylacetamide, N-methylpyrrolidone, dimethyl sulfoxide, dioxane, acetone triethyl phosphate and the like or acids such as, for example, acetic acid or formic acid have in general been used as solvents for the known membrane formation reaction.
The desired membrane type for the envisaged type of mass transfer can be obtained by means of a suitable composition of the polymer solution.